This invention relates to a correlator implemented type automatic equalizer utilizing a transversal filter, which is useful for constituting a ghost canceller or the like.
An apparatus which removes distortion components in electric signals by controlling the tap coefficients of a transversal filter is called an automatic equalizer and is widely used as waveform equalizers and echo cancellers in signal transmission lines. Recently, an attempt is being made to use a correlator implemented type automatic equalizer for eliminating ghost or multipath signals in a TV receiver or the like. In such automatic equalizer, following arithmetic operation for correlation is performed: ##EQU1## where k is integer (k=1, 2, . . . , n), x.sub.i denotes the differentiation components of an input wave form to be equalized, y.sub.i+k denotes an equalized output y.sub.i with a parameter k, sgn(y.sub.i+k) denotes the sign or polarity of y.sub.i+k, and d.sub.k denotes the result of the correlation operation for determining the tap coefficient of the transversal filter.
A digital transversal filter requires a low-pass filter for smoothing a digital dispersive filtered output of the transversal filter to obtain a pure analog filtered output. The low-pass filter is also effective to eliminate unnecessary higher frequency components contained in the filtered output. Such low-pass filter inherently involves a phase shift or time delay. Thus, the phase of signal y.sub.i+k is different from that of signal x.sub.i. For realizing accurate operation, however, the phase or the timing of occurrence of the signal x.sub.i must exactly be identical with that of the signal y.sub.i+k. From this, neutralization of the phase difference should be effected on the signaL x.sub.i to eliminate the timing discrepancy between the signals x.sub.i and y.sub.i+k.
In a prior art such as
Makino et al., "A Novel Automatic Ghost Canceller" PA1 IEEE Trans. CE-26,3,p629, Aug. 1980,
the signal x.sub.i is obtained from the input circuit of a transversal filter. Accordingly, where said timing discrepancy has to be positively removed, then a delay circuit is specially provided in the circuit line of the signal x.sub.i. Since the circuit components of this delay circuit are independent of the low-pass filter circuit, even though the timing discrepancy between these circuits is eliminated only at a specific condition, the timing discrepancy will unavoidably occur with time passage and/or temperature change. The timing discrepancy will cause the automatic equalizer to be unstable, to add derivative ghost components, or to undesirably oscillate. To escape such oscillation, the following relation should be always established: EQU .vertline..tau..sub.i -.tau..sub.0 .vertline.&lt;T/2 (2)
where .tau..sub.i denotes a delay time at the delay circuit, .tau..sub.0 denotes a delay time at the low-pass filter, and T denotes a transmission time delay per one tap of the transversal filter.
Further to avoid said addition of derivative ghost components, the difference .vertline..tau..sub.i -.tau..sub.0 .vertline. of equation (2) should be zero. According to the prior art, it is almost impossible to make the difference .vertline..tau..sub.i -.tau..sub.0 .vertline. always zero. In practice, even the establishment of equation (2) irrespective of time passage or temperature change is hard to obtain.